1. Field of the Disclosure
This invention relates to wet connectors for downhole use, which is to say, releasable connectors for electrical conductors which can be made and unmade in the fluid environment of a wellbore, particularly but not exclusively a hydrocarbon well.
2. Description of the Related Art
Wet connectors are used in hydrocarbon boreholes to releasably and remotely connect downhole equipment such as an electrical submersible pump (ESP), sensor or other tool to a conductor such as a power or signal line. The tool may be retrievably deployed in the borehole, e.g. on a wireline, or may be fixedly installed in the casing or other tubing in the wellbore. Similarly, the power or signal line may be retrievably suspended in the wellbore or may be fixedly installed on the casing or other tubing.
Hydrocarbon wells typically contain a mixture of electrically conductive fluids at elevated temperature and pressure, and since ESPs are typically powered at relatively high voltages, e.g. around 600V, the wet connectors are particularly vulnerable to failure when internal contamination of the connector by wellbore fluids leads to flashover between the conductors.
A wet connector typically comprises a male part comprising one or a group of plugs, and a female part comprising a corresponding number of sockets, the or each respective plug and socket having a single electrical contact or an array of contacts. Either the male or the female part may be arranged on the tool, with the other part being arranged on the power or signal line. For ESPs and other electrical tools running on a three phase power supply, the connector may comprise for example a single plug and socket having three axially spaced contacts, or a group of three plugs and sockets, each having a single electrical contact.
In order to exclude wellbore fluids from the connector, it is usual to occlude the bore of the socket with a retractable insert which is displaced by the plug. The sliding interface between the socket and the insert is protected by one or a series of annular seals known as wiper seals, hereinafter also referred to as wipers, which slidingly wipe contaminants from the surface of the plug as it enters the socket.
In practice it is found that as the plug enters the socket, contaminants clinging to the plug may travel past the or each wiper to form an electrically conductive path, leading to failure of the connector.
In order to reduce contamination, it is known for example from U.S. Pat. No. 4,997,384 and U.S. Pat. No. 4,825,946 to fill the socket with dielectric fluid which flushes the plug as it is inserted.
U.S. Pat. No. 4,767,349 discloses a wet connector in which a reservoir of dielectric fluid is arranged to energize an axial array of wiper seals as the plug is inserted, increasing the sealing force of each seal so as to assist in breaking the film of conductive fluid on the surface of the plug.
Although most wet connectors employ an array of wiper seals arranged along the insertion axis of the plug, which might be expected to effectively cleanse the plug of conductive fluids, it is found in practice that flashover still occurs between the contacts.
WO2010/122342 discloses a wet connector in which the plug is enclosed within a retractable sheath and may be repeatedly flushed by dielectric fluid expelled from a reservoir into the wellbore so as to cleanse the connector of contaminants. However, the reservoir of dielectric fluid may be exhausted by repeated flushing.
GB 2477214 A discloses a wet connect system in which a conductor is slidably housed in a conduit extending from the wellhead, through which a dielectric fluid may be pumped. Again, this is effective in excluding contaminants, but requires the installation of the conduit to the deployed depth of the wet connector.